It is well known that packed towers or columns are often useful for efficient gas/liquid contact to provide interactions, reactions and other mass transfer operations between gas and liquid. However, in some such operations the fluid passing through the packed bed contains suspended solid particles that tend to accumulate on the packing, eventually resulting in a reduction in gas/liquid volumetric flow rates and, in extreme cases, plugging of the tower.
Such plugging problems occur, for example, when a packed tower is used with liquids or gases that contain particulate material, or when particulates are formed within the tower as a result of a chemical interaction, reaction or the like, that precipitates solids as a result of mass transfer between the liquid and the gas. For example, in flue gas scrubbing that utilizes a liquid stream that includes an aqueous solution or suspension of limestone, or the like, to remove sulfur dioxide contained in a gas stream, a calcium sulfate precipitate is formed in the tower, and the resulting aqueous slurry causes plugging problems. Another example is a gas/liquid interphase oxidation process for removal of hydrogen sulfide from a gas stream that uses an oxidizing solution to effect oxidation of hydrogen sulfide to form elemental sulfur particles. Such particles may cause plugging of packed towers, requiring periodic shutdown of the process for a difficult and time consuming cleaning of the tower.
Some prior processes have used packed towers for contact of hydrogen sulfide (H.sub.2 S)-containing gases with an aqueous iron-chelate solution or a so-called "Stretford"-type alkaline vanadium ion-containing wash solution that converts H.sub.2 S to elemental sulfur (S.degree.). The elemental sulfur formed is entrained in the liquid mixture, and subsequently removed from the liquid, e.g., by settling or filtration. In this type of H.sub.2 S removal process, some of the sulfur settles on the packing material, thereby lowering gas and liquid flow rates and, ultimately, the sulfur plugs the tower. Mobile packing beds have been used in packed towers for sulfur removal from H.sub.2 S-containing gas streams but suffer from uneven gas distribution and channeling problems, particularly in larger cross-sectional area towers, resulting in decreased efficiency.
It is well known that two of the most important characteristics necessary for effective and efficient gas/liquid mass transfer in packed towers are 1) the tower must contain adequate passages for both the liquid and the gas streams without excessive liquid holdup or pressure drop; and 2) the tower must provide good contact between liquid and gas. The requirement of good contact between liquid and gas is the most difficult to meet, especially in large towers. Ideally, the liquid, once distributed over the top of the packing, flows in thin films over all the packing surface completely down the tower. Actually, the films tend to grow thicker in some places and thinner in others, so that the liquid collects into small rivulets and flows along localized paths through the packing. Especially at low liquid rates, and in the larger towers, some of the packing surface may be dry or covered with a stagnant film of liquid. This effect is known as "channeling" and is the main reason for poor performance in large packed towers. Similarly, a countercurrently flowing gas stream, especially in larger towers, sometimes tends to follow localized paths instead of being distributed across the entire cross-section of the tower, with a consequent increase in velocity through these localized paths, and this effect is known as "surging".
The use of mobile packing beds tends to decrease or eliminate packing material fouling, adds to the "channeling" and "surging" problems inherent in packed towers of relatively large cross-sectional area due to the tendency of the relatively light mobile packing material to collect along the sides and other localized areas of the tower, further decreasing the desired cross-sectional uniformity of gas and liquid flow. This reduces the overall efficiency in relatively large mobile bed gas/liquid contact devices. This problem has been recognized for many years but, to date, no adequate solution to the channeling and surging problems has been achieved. Others have provided vertical partitions in the columns between grids, to provide a plurality of partitioned packing beds between adjacent grids, thereby decreasing the cross-sectional area of each separate packing bed. This partitioning has aided the gas and liquid flow uniformity, but not completely satisfactorily, and has created other problems, such as the attendant difficulty in loading and unloading packing material and making sure that loading is done uniformly within each separate compartment.
The present invention solves this gas/liquid flow problem within mobile bed packed towers, by including at least one bed of mobile packing elements supported on a fluid-pervious grid disposed vertically below at least one bed of fixed packing elements supported on another fluid-pervious grid.